Sender-Constrained Tokens: How DPoP Solves the Bearer Token Crisis in Carrier Integration APIs

API downtime surged by 60% between Q1 2024 and Q1 2025, with average uptime dropping from 99.66% to 99.46%. This translates to approximately 10 extra minutes of downtime per week and close to 9 hours across a year. For carrier integration teams managing multi-carrier shipping APIs, this isn't just about inconvenient delays.

Your bearer tokens are sitting ducks. While APIs fail more frequently, traditional bearer tokens remain fundamentally vulnerable to theft through compromised log files, databases, and Authorization headers containing bearer tokens. The combination creates a perfect storm: unstable APIs plus authentication that anyone can replay if intercepted.

Tokens can be leaked through compromised client devices, insecure log files, person-in-the-middle attacks, or malicious browser extensions. Once leaked, a bearer token is a golden ticket for an attacker. When logistics saw the sharpest decline in API uptime as providers expanded their digital ecosystems to meet rising demand for real-time tracking, inventory updates, and third-party platform integrations, these vulnerabilities become business-critical issues.

Why Bearer Tokens Are Fundamentally Broken for Production Carrier APIs

Bearer tokens operate on a simple but dangerous principle: whoever presents the valid token to the resource server has access, which makes the token powerful and vulnerable. This "golden ticket" model breaks down quickly in real production environments.

Cross-Site Scripting (XSS) attacks inject malicious code that steals access tokens from browser's local storage or memory. With a bearer token, the attacker can now make API calls on behalf of the user. Consider a typical multi-carrier integration where your TMS platform stores tokens for FedEx, UPS, and DHL APIs. A single XSS vulnerability exposes all carrier credentials simultaneously.

Log file exposure presents another attack vector that integration engineers rarely consider. A developer might accidentally log an Authorization header containing a bearer token. If these logs are breached, the token is exposed. Similarly, if a database storing tokens is compromised, they are all at risk. How many times have you seen Authorization headers in application logs during troubleshooting?

Network proxy configurations in corporate environments create additional exposure points. Man-in-the-middle attacks can intercept bearer tokens during transmission, even over HTTPS if certificate validation fails or malicious certificates get installed on client devices.

Enter Sender-Constrained Tokens: The DPoP Solution

Demonstrating Proof-of-Possession (DPoP), standardized in RFC 9449, cryptographically binds access and refresh tokens to the specific application instance that requested them. Unlike bearer tokens that function as transferable artifacts, DPoP requires cryptographic proof with each request.

Here's how DPoP transforms the authentication model: When using DPoP, the client creates a "proof" using asymmetric encryption. The proof is a JWT, which includes the URI, the HTTP method of the request, and the public key. The client application requests tokens from the authorization server and includes the proof as part of the request.

The authorization server then binds a public key hash and the HTTP request information from the proof within the access token it returns to the client. This means the access token is only valid for the specific HTTP request.

If an attacker steals a DPoP-bound token, the stolen token is useless because the attacker doesn't have the client's private key required to generate the DPoP proof. The private key never leaves the client application, making token theft attacks ineffective.

DPoP vs. Bearer Tokens: Production Performance Reality Check

DPoP adds 2-10ms additional latency per request for signature generation and verification. While this might seem significant, consider the context: delays over 100ms can feel sluggish, while anything beyond 300ms risks losing user engagement. At 2 seconds, delays become noticeable, and by 5 seconds, most users abandon the application.

The DPoP overhead falls well within acceptable performance bounds for most carrier API integrations. Although this increases request size, the impact on bandwidth and latency is negligible due to the compact nature of JWTs and their short payloads. The server-side performance impact remains low. When implemented correctly, DPoP provides strong sender-constrained security with only marginal overhead.

Compared to alternatives like mTLS, DPoP offers significant advantages. Unlike mTLS, it does not rely on PKI infrastructure. DPoP is better suited for a public client such as a single-page application running in a browser. Since it does not rely on PKI, it is less cumbersome to deploy and much easier to implement.

Multi-carrier platforms are already implementing DPoP alongside traditional authentication methods. Companies like Cargoson, EasyPost, nShift, and ShipEngine must balance security improvements against integration complexity for their customers who need reliable access to carrier APIs.

Implementation Guide: Adding DPoP to Carrier Integration Flows

Adding DPoP to existing carrier integrations requires generating key pairs and creating signed proofs for each request. The process starts with generating an asymmetric key pair where the private key stays safely in the browser, while the public key can be sent to the server.

For each API request to carrier endpoints, you generate a DPoP proof JWT that includes:

• HTTP method (GET, POST, PUT, DELETE)
• Target URL of the carrier API endpoint
• Timestamp to prevent replay attacks
• Unique identifier (jti) for the request
• Public key for verification

Before the client sends any requests, it generates a DPoP proof — a signed JWT that includes the public key, some metadata (HTTP method, URL), and a signature. This proof gets attached to requests in the DPoP HTTP header alongside the Authorization header containing the access token.

Server-side verification involves checking the JWT signature against the embedded public key, validating the HTTP method and URL match the request, and ensuring the token's key binding matches the proof's public key. Servers may temporarily store recently used jti values to prevent replay attacks. This storage is typically short-lived, often no more than 30 to 60 seconds - and is easily managed by in-memory caches or distributed cache layers.

The Multi-Carrier Security Challenge: Platform Considerations

Multi-carrier shipping platforms face unique challenges when implementing DPoP. Each carrier API may have different authentication requirements, migration timelines, and compatibility levels with sender-constrained tokens. UPS is replacing its entire existing API infrastructure, while FedEx isn't just updating its API; it's championing a digital transformation. Both carriers had migration deadlines that got pushed back multiple times as companies struggled to adapt.

Platform providers must balance security improvements against backward compatibility. Companies like EasyPost, nShift, Cargoson, and ShipEngine build redundancy into their systems that individual carriers can't match. These platforms can implement DPoP gradually, starting with carriers that support sender-constrained tokens while maintaining bearer token support for legacy integrations.

The authentication complexity multiplies when managing tokens for FedEx Express, FedEx Ground, UPS, DHL Express, DHL eCommerce, DSV, and regional carriers simultaneously. Each requires separate key pairs and proof generation when using DPoP, but the security benefits outweigh the implementation overhead.

Sender-constrained tokens are suitable implementations for environments with high security requirements such as financial-grade APIs or Open Banking. With support for DPoP, Kong enforces proof-of-possession checks for both methods of sender-constrained tokens.

Future-Proofing Carrier Integrations: Beyond DPoP

AI agents hitting carrier APIs represent the next authentication challenge. These automated systems require predictable, fast response times and consistent error handling. Traditional bearer tokens become even more problematic when AI systems cache and reuse credentials across multiple sessions or contexts.

DPoP provides a foundation for AI-safe authentication because even if a token leaks, it cannot be misused or replayed by an unauthorized party. This makes token-based authentication far safer across environments where secure storage is difficult, such as browser apps, mobile devices, and distributed application clients.

There is a clear industry trend toward widespread adoption of PoP mechanisms like DPoP and mTLS as standard approaches for securing API tokens. This shift reflects growing awareness of the inherent risks of traditional bearer tokens and a proactive move toward cryptographically binding tokens to their intended users or clients. As these trends continue to develop, organizations should stay informed about evolving standards.

The OAuth working group continues developing enhanced security standards. Future specifications may include additional proof-of-possession mechanisms optimized for different client types and network conditions. Carrier integration teams should monitor these developments while implementing DPoP as a current best practice.

Start by auditing your current bearer token implementations across all carrier integrations. Document which carriers support DPoP or similar sender-constrained authentication. Plan a gradual migration that prioritizes high-value or high-risk integrations first. The 2-10ms latency overhead of DPoP is negligible compared to the business impact of compromised carrier credentials.